The present invention relates to a spread spectrum communication apparatus used in a digital cellular radio system adopting a code division multiple access (CDMA) method as a radio channel access method.
In recent years, the spread spectrum communication method immune to interference and disturbance attracts attention as one of communication methods applied to mobile communication systems. The spread spectrum communication method is used mainly to implement a cellular radio system adopting the CDMA method.
In the cellular radio system adopting the CDMA method, a transmitting device modulates digitized audio data and image data by using a digital modulation method such as a PSK modulation method, then converts the modulated data to be transmitted to a wideband baseband signal by using a short code, up-converts the wideband baseband signal to a signal of a radio frequency band, and transmits the up-converted signal. The short codes are generated by shifting a phase of a spreading code having a periodicity. For example, a pseudo random noise code (PN code) is used as the spreading code. On the other hand, a receiving device down-converts the received radio frequency signal to a received signal of an intermediate frequency or a baseband frequency, despreads the down-converted signal using same short code used in the transmitting device, then conducts digital demodulation by using a digital demodulation method such as a PSK demodulation method, and thus reproduces received data.
In other words, in the CDMA method, different short codes are assigned to radio communications between a plurality of mobile stations and a base station to ensure channel separation among radio communications.
When a mobile station starts communication in such a system, it is necessary to establish synchronization for the short code of a pilot signal transmitted by a base station. As its synchronization acquisition method, the following method, for example, has been proposed heretofore.
A PN code has a length of, for example, 2.sup.15 (one period=26.6 msec). Each base station generates a PN code to produce the short codes from a preset different chip phase every radio area managed by its own station. The chip phase whereat generation of the PN code is started is called offset.
When power is turned on, each mobile station first generates a short code from its own PN code generator while successively shifting the chip phase from an arbitrarily set initial reference phase. By monitoring a correlation value between this short code and the short code of the pilot signal broadcast from a base station, the mobile station searches for the offset of the PN code used by the base station. If the offset of the PN code used by the base station is acquired by this search, then a sync channel is received and a system identification (SID) for identifying the mobile communication network is checked. If this SID is one corresponding to a home system in which its own station was registered beforehand, then the mobile station proceeds to a waiting state.
However, such a short code synchronizing method heretofore proposed has the following problem to be solved. Whenever power of the mobile station is turned on in the conventional short code synchronizing method, the mobile station generates a PN code from an arbitrarily set initial phase and conducts the operation for acquiring the synchronization of the pilot signal as described above. Even in the case where, for example, the power of the mobile station is temporarily turned off and the power of the mobile station is turned on again without moving from that radio area, therefore, the mobile station necessarily generates a PN code from the initial reference phase and acquires the synchronization of the pilot signal. As a result, it takes a long time to acquire the synchronization of the short code, depending upon the positional relation between the offset of the PN code to be acquired and the initial reference phase of the mobile station.